NullReferenceException in my code, need help

Need some help cause plugin is spamming ex. in the console-

Code:

(23:34:53) | [Oxide] 23:34 [Error] Exception while calling NextTick callback (NullReferenceException: Object reference not set to an instance of an object)
(23:34:53) | [Oxide] 23:34 [Debug]   at Oxide.Plugins.HuntPlugin+<OnStructureUpgrade>c__AnonStorey1.<>m__0 () [0x00000] in <filename unknown>:0
  at Oxide.Core.OxideMod.OnFrame (Single delta) [0x00000] in <filename unknown>:0

Heres the Code Strip.

Code:

object OnStructureUpgrade(BuildingBlock buildingBlock, BasePlayer player, BuildingGrade.Enum grade)
        {
            NextTick(() =>
            {
                if (buildingBlock.grade != grade) return true;
                var items = buildingBlock.blockDefinition.grades[(int) grade].costToBuild;
                var total = 0;
                foreach (var item in items)
                    total += (int) item.amount;
                var experience = (int) Math.Ceiling(UpgradeBuildingTable[grade]*total);
                ExpGain(FindRpgInfo(player), experience, player);
            });
            return null;
        }